1. Technical Field
The present invention relates to a method of driving a transistor, a driving element using the same, and a display panel and a display apparatus having the driving element.
2. Discussion of the Related Art
Current liquid crystal displays (LCDs) have various characteristics, such as high luminance, high efficiency, uniform luminance, long lifetime, thinness, lightweight, low cost, etc. An LCD include a blacklight to display images. One type of display device that does not include a blacklight is an organic electro luminescent display (OELD).
The OELD displays an image using the electro luminescence of an organic material or polymers. The OELD has various characteristics, such as thinness, low cost, a wide viewing angle, light luminescence, etc.
The OELD also includes an active matrix type OELD and a passive matrix type OELD. The active matrix type OELD includes a switching element disposed in a unit pixel. The passive matrix type OELD does not include a switching element disposed in a unit pixel.
FIG. 1 is a circuit diagram showing a conventional OELD 100. FIG. 2 is a timing diagram showing a data voltage (Vd) applied to a unit pixel of the OELD 100 of FIG. 1. Referring to FIGS. 1 and 2, a unit pixel of the OELD 100 includes a switching element (QS), a driving transistor (QD), a storage capacitor (CST) and an organic electro luminescent element (EL).
The luminescence of the OELD 100 is less than that of a display such as a cathode ray tube (CRT) display. The efficiency, however, of the active matrix type OELD is greater than that of the passive type OELD, therefore the active matrix type OELD is frequently used in the OELD 100.
The mobility of a polysilicon is greater than that of amorphous silicon. The amorphous silicon does not include a positive-type (P-type) transistor, and as the amorphous silicon is fragile it is subject to a bias stress. Therefore, the OELD 100 may include a polysilicon transistor, even though it is more expensive than an amorphous silicon transistor. The OELD 100, however, may also include the amorphous silicon transistor, which includes a driving circuit having a negative-type (N-type) transistor.
Current flowing through the organic electro luminescent element EL of a current driving OELD may be adjusted to display a gray color. In order to control the current flowing through the organic electro luminescent element EL in response to a data signal applied from an exterior to the organic electro luminescent element EL, a thin film transistor (TFT) is serially connected to the organic electro luminescent element EL to apply the data signal to a gate electrode of the driving transistor QD, thereby controlling a channel conductance in response to a gate-source voltage (Vgs) of the driving transistor QD.
When the driving transistor QD includes the P-type transistor, a bias line (VL) serves as a source electrode so that the amount of the gate-source voltage Vgs applied to the driving transistor QD is determined by a data voltage applied to the gate electrode of the driving transistor QD through a data line (DL).
When the driving transistor QD includes the N-type transistor, the organic electro luminescent element EL serves as a source electrode and the voltage applied to a node electrically connected to the driving transistor QD and the organic electro luminescent element EL is unstable. The voltage applied to the node is also dependent on data from a previous frame. In addition, the range of the gate-source voltage Vgs applied to the driving transistor QD is narrower than the range of the data voltage applied to an active region including the driving transistor QD and the organic electro luminescent element EL from an exterior to the driving transistor QD. Therefore, the OELD 100 may include the driving circuit having the P-type transistor.
When the data voltage having the same polarity is applied to the gate electrode of an amorphous silicon TFT for an extended period, the output characteristics of the amorphous silicon TFT deteriorate. In other words, when the data voltage having the same or constant polarity (e.g., a positive polarity) as shown in FIG. 2 is applied to the gate electrode of the driving transistor that controls the output current in response to the gate voltage for an extended period, the characteristics of the amorphous silicon TFT deteriorate.
The amount of the output current also changes in response to the variation of the characteristics of the amorphous silicon TFT, resulting in a malfunction of the driving transistor. The malfunction increases in proportion to an operation time and, therefore, the lifetime of the amorphous silicon TFT is decreased.
In order to control the organic electro luminescent element EL with an output current, a predetermined voltage is applied to the gate electrode of the amorphous silicon TFT. The voltage level applied to the gate electrode may be changed, but a constant voltage having a positive polarity may be applied to the source electrode or the drain electrode.
When the characteristics of the amorphous silicon TFT deteriorate, a charge is injected into an interface between a gate insulator and the gate electrode and the charge is trapped between the gate insulator and the gate electrode and a defect is formed on an amorphous silicon layer, thereby changing the threshold voltage (Vth) and the output current. Accordingly, the injected charge and the resulting defect increase in proportion to the operation time of the amorphous silicon TFT. Thus, there is a need for reducing the effects of an injected charge in an amorphous silicon TFT.